THE VIBRANT ECOSYSTEM INSIDE THE HUMAN GUT

DOES MORE THAN UST DIGEST FOOD.

GUT REACTION BY SARAH C.P. WILLIAMS of the Yale School of Medicine, who also studies the microbiota. “And that’s not just a metaphor; they really shape our biology in many ways.” While the studies of Knight, Gewirtz, and Ley hint at the importance of the microbiota in human health, scientists have more questions than answers. After all, most of the bacteria swarming in our guts remain unidentified. Many cannot survive outside the complex environment of the intestines and so are difficult to culture in the lab. Plus, the microbiota of any one person is shaped not only by genetics but also by diet and envi- ronment, meaning that they change over time, complicating experiments even more.

genetics of the fat and skinny mice: identical. Their ages: all the CENSUS TAKERS same. The food in front of them: the same bland pellets in every Knight’s background is in ecology, studying the interplay of cage. But the billions of bacteria teeming through each mouse’s plants and animals in natural environments. He views the human intestines: vastly different. body as another environment to study, in the same way he would The variation in gut bacteria between lean and obese mice study a jungle, marsh, or arctic tundra. Except, he says, bacteria isn’t just a consequence of their health, it’s a cause, according are a lot more convenient to study. “Instead of having to do field to work by Rob Knight, an HHMI early career scientist at the seasons every year for a decade, you can draw out four quadrants University of Colorado at Boulder, Emory University patholo- on your forearm and have your field season in your office in gist Andrew Gewirtz, and their colleagues. Transferring gut fifteen minutes.” bacteria from obese mice with insulin resistance and metabolic The first thing an ecologist does in an environment is take syndrome to healthy mice makes the healthy mice develop meta- a census of what’s there. So that’s Knight’s first task when it bolic syndrome, they found. Giving the mice antibiotics before comes to the intestines, and it’s the goal of microbiota research- the bacterial transfer can prevent the syndrome. Moreover, the ers worldwide. Rather than a butterfly net and microscope, they researchers discovered that the changed microbiota doesn’t just use cutting-edge DNA sequencing technology to get snapshots affect molecules inside the mouse gut, it also affects outward of the genetic makeup—called the microbiome—of individuals’ behavior: the mice eat more than their healthy counterparts who particular microbial mixtures.

didn’t receive transferred bacteria. Knight takes samples of bacteria—from skin, feces, and the “Out of all our studies on obesity and microbes, this was intestines of mice or humans—and sorts out every copy of one the most shocking thing to me,” says Ruth Ley, one of Knight particular gene encoding a bacterial ribosome. The ribosome and Gewirtz’s collaborators in data analysis at Cornell University. is the cellular factory that produces proteins. It varies enough “To have microbes actually affecting behavior.” among bacteria that its sequence can place a bacterium (even These researchers and a growing number of other scientists a previously unknown one) into what’s called a phylotype— study the microbiota—the microbes that make our bodies their essentially, “a spot on the tree of life,” says Knight. Similar homes. While bacteria call to mind disease-causing germs that ribosomes are from similar bacteria and therefore appear on require antibacterial soaps and drugs to exterminate them, the nearby branches of the tree. Knight originally developed a bacteria in healthy intestines are relatively tame. In fact, animals computational method, called UniFrac, to study the ribosomal from snakes to mice to humans wouldn’t survive without them. differences among bacterial communities in sediment, ice, and They turn food into energy, synthesize vitamins that their hosts’ water. Now, his team is using the same technique to tackle medi- bodies can’t produce, and engage in a complex and vital interplay cal questions. with the immune system. In a 2009 Science paper, Knight’s lab group reported Researchers are also showing that the composition of a per- using UniFrac to analyze bacteria from 27 sites on the bod- son’s microbiota can predispose him or her to diseases—from ies of nine individuals, collected on four different dates. He asthma and allergies to cancers, infections, and inflammatory found that bacterial communities varied drastically from per- bowel disease. son to person and changed to a lesser extent in one person “Our individuality is not just us genetically, it’s us plus all the over time. Only 3 percent of bacterial phylotypes appeared in microbes we carry,” says HHMI investigator Ruslan Medzhitov all individuals on all occasions. In a separate paper, Knight

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and collaborators showed that people’s skin bacteria differ With that in mind, he wants to use UniFrac to observe the enough that you leave behind unique molecular fingerprints complicated dynamics between human health and the diversity on everything you touch—a finding that could change foren- of phylotypes in the gut. sic science. While Knight’s initial goal is to use UniFrac to characterize SEA OF MICROBES the microbiomes of healthy individuals, he says it will take the Martin Blaser, a doctor of infectious diseases and chair of medi- field only so far. Significant findings in ecology often come not cine at New York University Langone Medical Center, thinks from studies of healthy environments but from observations of there’s already a disturbance going on in human guts that’s not disturbances within a community. unlike the disappearance of wolves from Yellowstone. From early “If you went to Yellowstone and ground up a cubic mile human history until the 20th century, Blaser says, a bacterium of it and analyzed the DNA, you wouldn’t find a lot of wolf called Helicobacter pylori was universal. DNA,” says Knight. But scientists know that wolves are a “You can actually trace human migration by looking at species crucial to maintaining Yellowstone’s diversity because variations in Helicobacter,” he says. Today, Helicobacter is still of the radical changes that took place when they were removed ubiquitous in developing countries around the world. In the from Yellowstone in the early part of the 20th century. Elk United States, though, fewer than 6 percent of children have populations skyrocketed and the condition of the woodlands Helicobacter in their mix of gut bacteria. And in a series of recent deteriorated. In 1995, scientists reintroduced wolves to Yellow- studies, Blaser showed that children lacking Helicobacter are stone and have since observed the elk population stabilize at a more likely than their peers to develop child-onset asthma. While healthier number. the presence of Helicobacter predisposes people to ulcers, he In the gut, Knight says, there could be bacteria that, while not thinks the disappearance of the microbe could explain not only abundant, keep the populations of other microbes in check. rising rates of asthma but also allergies and obesity.Medzhitov: Chris Jones Finlay: Birthe Piontek

tion, weak spots, or cancer—they replace it with a new colon, tant to Salmonella infection. Other findings, by Finlay and his built from other nearby tissue. At first, this pouch is void of bac- colleagues, suggest that shifts in the microbiota caused by differ- teria. Gradually, a microbial community develops. But in almost ent antibiotics can weaken the immune system. half of all pouches, symptoms of colitis develop. So Schmidt and “When researchers compare mice with different degrees of his collaborators are following patients with new colons and track- susceptibility to disease, they’ve always searched the mouse genes ing the development of the microbiota in each case. They hope for the explanation and not found much,” Finlay says. “Now to discover how the intestinal flora keeps some colons healthy and we’re learning that’s because the difference isn’t in the mouse others prone to infection and inflammation. genes, it’s in the microbiota.” Schmidt, like Knight, comes at the microbiota with an ecol- ogy background. His expertise is in soil ecology, and soil has NOT SO BLACK AND WHITE surprising similarities to the gut, he says. Both environments In any kind of census, it’s necessary to group individuals into are low in oxygen. So the techniques he developed to cultivate categories that oversimplify their differences. In a human census,

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that means checkboxes that reduce people to race, gender, stimulated immune responses, helped detoxify compounds theincome, and marital status. In the microbiota census occupying mice ate, stimulated the growth of new blood vessels, allowedscientists’ minds, it means the temptation to group gut bacteria proper tissue development, and performed countless meta-into good and bad, pathogens versus commensals (microbes that bolic tasks.don’t cause disease). But this process hardly paints a full picture When she moved to her own lab, Hooper took another lookof what’s going on in our intestines, says Yale’s Medzhitov. at the plethora of genes that shot up in expression levels when “The difference between commensals and pathogens is not a mouse was first exposed to gut bacteria. She chose one to studythat they are two very different types of microbes,” Medzhitov in more depth. Her lab quickly discovered that it was the geneexplains. In fact, microbes considered commensals in one organ- for a protein dubbed RegIIIƢ, and it had a rare job for a protein:ism’s gut, or in one situation, can act pathogenically in a host it’s an antibiotic. She’s gone on to show that RegIIIƢ can killwith a compromised immune system or in a different organism. bacteria by drilling a hole in their outer layers, allowing their“So the distinctions between the two are in many ways arbitrary,” contents seep out. Hooper speculates that RegIIIƢ may helpMedzhitov says. to shield the intestinal epithelium from the bacteria sloshing In reality, each bacterium in our gut falls along a spectrum around inside the gut.between pathogenic and commensal. Medzhitov thinks that “The epithelium itself is a barrier, but you want to minimizeinstead of lumping bacteria into these extremes, based on their even having bacteria attach to that,” says Hooper. “So these anti-outcomes (disease or health), scientists should focus on how each microbial proteins, and probably many other immune molecules,bacterium interacts with its host—the human body. likely help to set up a secondary barrier.” Humans have intricate immune systems exquisitely tuned to In patients with inflammatory bowel disease, more bacterialidentify intruders. Medzhitov studies a class of receptors—called cells reach the intestinal lining, indicating that the chemical bar-Toll-like receptors—that recognize invading bacteria and signal rier is inadequate—one hint toward a cause of this chronic disease.the immune system to act. His lab has found that Toll-like recep-tors recognize bacteria that could do harm to the body, and they PERSONALIZE THE GUTalso help keep the intestinal microbiota balanced by detecting Though there’s a growing body of evidence that links variationsbacteria that are less virulent. in the microbiota to diseases—from Knight’s studies on meta- “There’s something about the environment of the gut that bolic syndrome to Blaser’s work on asthma and Schmidt’s colitiscontrols this interaction so that, under healthy conditions, Toll- research—the mechanisms of these links are still too sketchy tolike receptors sense commensals and don’t react to them as translate them into clinical medicine.harmful,” says Medzhitov. Of course, if those same tame bacteria A 2009 paper in the Proceedings of the National Academy ofsneak out of the gut into the bloodstream—during surgery, for Sciences by Jeremy Nicholson at Imperial College London andexample—the receptors will respond with fury, leading to danger- collaborators at the drug company Pfizer found that variationsous inflammation and sepsis. Medzhitov wants to know what it is in one type of gut bacteria lead to differences in how peopleabout the intestines that keeps Toll-like receptors in check. metabolize acetaminophen (Tylenol) and different propensities And that’s far from the only way the immune system inter- for liver toxicity. The authors proposed that “assessing the effectsacts with gut bacteria. HHMI investigator Lora Hooper, at the of microbiome activity should be an integral part of pharmaceuti-University of Texas Southwestern, is fascinated by the subtleties cal development and of personalized health care.”of the interaction. The concept resonates strongly with Knight. The vast variation “I’ve been studying this for 15 years and it’s still not clear to he’s seen between individuals’ microbiomes leads him to thinkme—how can you have a hundred trillion bacteria in your gut that gut bacteria will be targeted with drugs, or personalized con-and you don’t get sick?” says Hooper. coctions of healthy bacteria, in the future. When she first started studying the microbiota, as a post- “In terms of developing personalized medicine,” he says,doctoral fellow in the lab of Jeffrey I. Gordon at Washington “it seems like it makes more sense to develop medicines based onUniversity in St. Louis School of Medicine, Hooper began work- the microbiome, where the variation is so great, rather thaning with germ-free mice. These mice are raised from birth in on the human genome, where the variation is so little.”sterile environments—they eat sterilized food, live in germ-free For now, though, patients and doctors are stuck at the impassebubbles, and interact only with other germ-free mice. These between knowing (or guessing) the cause of a disease and havingcleaner-than-clean mice allow researchers to study the effects of a treatment. Someone can blame their diabetes or inflammatoryindividual bacteria strains in a simple system. bowel disease on the churning mass of bacteria that lives inside Hooper and Gordon’s first experiments with germ-free mice their intestines, but there’s no magic pill to change the dynamicsgave them a glimpse at some of the jobs of gut bacteria: they of that complicated world of the human microbiome. W